C.F.J. Flipse

Atomic-scale friction on diamond(111) studied by ultra-high vacuum atomic force microscopy

Abstract The frictional properties of a diamond(111) single-crystal surface have been studied using an ultra-high vacuum atomic force microscope, while the presence or absence of hydrogen on the surface was monitored by low-energy electron diffraction (LEED). We have observed, for the first time, atomic-scale stick-slip features of a silicon tip on the hydrogen-terminated (1 × 1) diamond(111) surface. The distance between the stick-slip rows, measured perpendicularly to the rows, is typically 2–3 A, consistent with the lattice parameters of diamond. Removal of the hydrogen from the surface, indicated by a change in LEED pattern from (1 × 1) to (2 × 1), gives rise to enormous stick-slip features at larger scale. The average friction coefficient on the hydrogen-free surface is found to be more than two orders of magnitude larger than on the hydrogen-terminated surface for loads up to 30 nN.

Ordering of organic molecules on passivated reactive substrates: PTCDA on O-p(2x2)-Ni(1 1 1)

The ordering of an organic molecular layer on a ferromagnetic substrate is studied using scanning tunnelling microscopy. Highly ordered layers of perylene-tetracarboxylic-dianhydride (PTCDA) were prepared by vacuum sublimation on an oxygen precovered Ni(1 1 1) surface. The structure of thin layers of PTCDA deposited at room temperature was investigated as a function of growth rates and thickness. It is demonstrated that oxygen passivation reduces the reactivity sufficiently to lead to well-ordered overlayers of PTCDA. For thin films grown at low deposition rates, a herringbone-like structure has been observed. This structure is consistently observed in the islands with typically 100 nm in diameter and 1–2 ML thickness. Depositing thicker films at higher deposition rates results in polycrystalline islands. Within the polycrystalline islands two distinct stripe-like phases are observed in domains with a lateral size of typically 10 nm. The potential impact of these results for organo-metallic spintronic devices is addressed.

REACTION OF NANOMETER-SIZED CU PARTICLES WITH A SIO2 SUBSTRATE

The thermal stability of nanometer-sized Cu particles on a 400–500 nm thick SiO2 layer on top of a Si(100) substrate was studied after annealing in ultrahigh vacuum up to 620 °C. Atomic force microscopy, low-energy ion scattering, Rutherford backscattering spectrometry, and Auger electron spectroscopy measurements clearly show that Cu-silicide islands are formed. A direct reaction of Cu with the SiO2 support is assumed, which is facilitated by a fairly strong metal-support interaction and by the wetting behavior of the silicide islands. Exposure to air at room temperature results in regeneration of the annealed Cu/SiO2 system.

Nanocluster formation by spin coating : quantitative atomic force microscopy and Rutherford backscattering spectrometry analysis

A recently developed spin coating method has been employed to produce a homogeneous distribution of nanometer‐sized metal clusters onto a flat oxidic support. The particle size and distribution, and the total amount of material deposited has been studied by comparing the results of atomic force microscopy (AFM), Rutherford backscattering spectroscopy (RBS), and the appropriate hydrodynamic deposition equations. It is shown that the AFM is capable of producing a three‐dimensional image of the surface which enables the particle number density and particle heights to be accurately determined. However, it is clear that as a result of tip convolution effects the particle diameter cannot be accurately determined. Using a hemispherical particle model the amount of material deposited during spin coating can be calculated from the AFM images. This calculation is shown to be accurate to approximately 50% in comparison with the results obtained from RBS. In contrast, it is shown that for a copper acetate precursor t...

Electromigration of single metal atoms observed by scanning tunneling microscopy

The authors show in this letter that single metal atoms on a Ni(111) surface can be pushed by electromigration forces from a scanning tunneling microscope tip. This repulsive interaction is observed over a length scale of 6nm. While for voltages above −300mV the atoms are pulled by the microscope tip, the atoms are pushed away below this threshold. This migration is explained by a resonant scattering of strongly correlated electrons. At small voltages chemical forces are pulling the atom, while for larger voltages the atomic manipulation is assisted by the tunneling current.

An AFM investigation of the deposition of nanometer-sized rhodium and copper clusters by spin coating

Abstract We have used atomic force microscopy to investigate the deposition of nanometer-sized clusters by spin coating and it is shown that it is possible to produce a homogeneous distribution of nanometer-sized Cu and Rh particles using this technique. However, the formation of particles with a uniform size and distribution is not only dependent on the solute concentration and spin frequency, as has been discussed previously, but also on a number of other factors, including: atmospheric humidity, solvent properties and the chemistry of the solute. In the case of the simple salts, Cu(NO 3 ) 2 and RhCl 3 dissolved in ethanol, particles precipitate out of the solution during spin coating and deposit onto the substrate. However, AFM and XPS analysis reveals that the use of a Cu(acetate) 2 precursor results in the formation of a layer of Cu(acetate) 2 on the substrate. This behaviour is attributed to the existence of a larger metastable super-saturated region in the Cu(ac) 2 solution resulting from the presence of the acetate ligands. The layer of Cu(ac) 2 is observed to form particles on calcination, the particle size and distribution being sensitive to the calcination rate. Possible factors responsible for the ramp rate sensitivity are discussed. AFM imaging of the surface following oxidation and reduction of the larger Rh particles indicate that these particles break up as a result of the treatment, where this behaviour is consistent with previous studies. Following this treatment, or after direct reduction in H 2 , the Rh particles are observed to exhibit a particle-substrate interaction, indicated by the inability of the AFM to sweep the particles across the surface. A similar behaviour is also determined to occur following oxidation of the Cu particles.

Au structures on Si(111) studied by spectroscopic ellipsometry and optical second harmonic generation

Abstract Two optical techniques, spectroscopic ellipsometry (SE) and second harmonic generation (SHG), have been used to follow the initial stages of ordered growth of Au on the Si(111) surface under ultra-high vacuum conditions. Comparison of the linear and nonlinear optical response, as a function of coverage, shows that the nonlinear response is more sensitive to submonolayer structural changes, for this system. Differences in the linear optical response of amorphous and ordered interfaces indicate the necessity for more sophisticated approaches to microscopic modelling of ultra-thin film growth.

Quantum-sized Au structures on Si(111) studied by spectroscopic ellipsometry

Abstract Spectroscopic ellipsometry has been used to follow the initial stages of the growth of Au on both singular and vicinal Si(111) surfaces. The imaginary part of the pseudo dielectric function shows an distinctive sharp peak at 2.7 eV, at the point where the single-domain pseudo-5 × 1 low energy electron diffraction pattern is strongest for Au growth on the stepped Si(111) surface. The singular system behaves differently. The unusual optical behaviour of the Au-induced 5 × 2 reconstruction is attributed to the recently-discovered quasi one-dimensional electronic structure of this system.

Growth of organic molecules on ferromagnetic metallic substrates : a combined STM/AFM study

The growth of large aromatic molecules on the 3d ferromagnetic transition metals is studied, in view of application into hybrid organo metallic electronic devices in which the spin degree of freedom is exploited. It has been previously shown that the ordering of organic molecules on ferromagnetic metallic substrates is mostly hindered due to the high reactivity of the substrate, which can even lead to dissociation. In this work we demonstrate that molecular ordering can be induced when the substrate is passivated or when the proper molecule‐substrate combination is chosen. Structural properties of PTCDA and pentacene films grown on well‐oriented substrates as well as on polycrystalline substrates are presented.

Development of a Near-Field Magneto-Optical Microscopy for Studying Ultrafast Magnetization Dynamics

Apertureless magneto‐optical near‐field microscopy is developed for studying sub‐picosecond spindynamics at nanometer spatial scale. Polarization modulation and tip vibrations are implemented. Polarization responsivity of the tip‐induced scattering is demonstrated using polarization modulation and tip vibrations. A near‐field magneto‐optical contrast is achieved for a thin ferromagnetic film with in‐plane magnetization; however, its interpretation is shown to be nontrivial. Distribution of the evanescent field is measured vs. tip‐sample distance. Interference of the near‐field with the far‐field scattered light is found at distances ca. 300 nm and more, and strong tip‐surface interaction that quenches the detected scattering is found at distances below 20 nm.